Thermionic emission is defined as the outflow of electrons into vacuum from a substance as a result of heat. The phenomenon of thermionic emission is related to the work function of the material which is the minimum energy needed to remove an electron from the Fermi level of the substrate to infinity. Because the work function of lanthanum hexaboride (LaB.sub.6) is relatively low, it has been used in the past as an electron source, such as an electron beam emission cathode. LaB.sub.6 has been used either as a single crystal of LaB.sub.6 or in the form of a sintered rod. The single crystal tip provides better emission qualities, i.e. higher current in smaller areas at lower temperatures, but is more expensive than the sintered rod. However, both the single crystal and the sintered rod must be indirectly heated, which complicates the engineering and design.
In the past it has also been proposed to produce an electron beam emission cathode by coating metal filaments with LaB.sub.6 through use of an electrophoretically deposited coating. While filaments of this type are relatively simple in design, they are mechanically fragile and prone to spalling of the coating which reduces the service life of the coated filament. As a further disadvantage, electrophoretically coated filaments have a relatively rough surface which can result in stray emissions which detracts from the performance of the cathode.
It is also known to coat materials by radio frequency magnetron sputter deposition. Sputtering is a thin film deposition process that is based on the bombardment of a target of a source material with positive ions from a plasma. The plasma is formed by applying a high voltage between the cathode, which is the metal to be deposited, and the anode or substrate in a high vacuum system, backfilled with an inert sputtering gas, such as argon. Positive ions in the plasma are attracted to the negative potential on the cathode or target and cause ejection of the target material by momentum transfer. The ejected material is deposited onto the substrate.
Radio frequency sputtering has been used for the deposition of thin films in the production of devices, such as capacitors, resistors, transistors, and the like.